Sheet supply trays are commonly used with imaging devices, such as copiers, printers, facsimile machines and the like for supplying cut sheets to these devices in order to form an output image on each sheet. In this regard, a column of sheets are loaded in the tray which is then received in the imaging device. A roller mechanism belonging to the device engages the column of sheets to individually feed the sheets into a sheet guide path leading to a print head located in the device. The print head prints the output image on each sheet.
However, it has been observed that, occasionally, the sheets will inadvertently "multiple-feed" into the guide path. That is, two or more sheets will inadvertently feed from the supply tray and become jammed in the guide path. When this occurs, the device becomes inoperable until the jammed sheets are removed from the guide path. Alternatively, two or more of the sheets may be transported along the guide path and encounter the print head to become jammed at the print head, which may undesirably attempt to print the image on the multiple sheets jammed at the print head. In either event, the jammed sheets must be removed, usually by hand, resulting in lost time to remove the jammed sheets and wasted sheet stock. Therefore, a problem in the art is multiple-feeding of sheets from the sheet supply tray.
A device for separating and supplying sheets one by one from a sheet stack is disclosed in U.S. Pat. No. 5,398,108 titled "Sheet Supplying Apparatus Having a Plurality of Pressurizing Means" issued Mar. 14, 1995 in the name of Kazuyuki Morinaga, et al. This patent discloses an arrangement whereby, when a large number of sheets are stacked on an intermediate plate belonging to the arrangement, a plurality of pressurizing means are used to bias the intermediate plate with a force such that each sheet engages a feed roller with a predetermined pressure. On the other hand, when the number of the stacked sheets is small, a smaller number of the pressurizing means are used to bias the intermediate plate with a smaller force, so that each sheet engages the feed roller with substantially the same predetermined pressure. According to this patent, this arrangement allows the sheet supply pressure to be maintained substantially within an ideal range for eliminating poor sheet supply regardless of the number of sheets stacked on the intermediate plate. Moreover, this patent discloses a pair of separation pawls cooperating with the pressurizing means, which separation pawls are positioned for separating the sheets, one by one. However, each of Morinaga's, et al. separation pawls appear to be flat across its entire surface for making contact with a corner portion of the sheets. Therefore, it appears that the entire flat surface of each separation pawl is in contact with each sheet being fed, so that when the sheet begins to move out of the device the corner portion is dragged across the entirety of the flat surface. Thus, this flat configuration of each separation pawl causes the corner portion of the sheet to remain in contact with the separation pawl for a relatively longer time, thereby allowing frictional forces between the sheet and separation pawls to retard release of the sheet from the device. This in turn may allow a second sheet to begin feeding before the relatively slow-moving first sheet is completely released from the Morinaga, et al. device. Of course, this relatively slower feeding of the sheets could lead to the previously mentioned multiple-feeding problem. Hence, this patent does not appear to disclose a suitable sheet feeding apparatus having separation pawls adapted for reliably releasing the sheets from the intermediate plate to avoid multiple-feeding the sheets.
Therefore, what has long been needed is a sheet feeding apparatus and method for reliably feeding sheets from a column of sheets, one sheet at a time, in a manner that avoids multiple-feeding.